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Date: Mon, 6 Jul 2020 09:26:33 -0700
From: "Paul E. McKenney" <>
To: Peter Zijlstra <>
Cc: Marco Elver <>,
	Nick Desaulniers <>,
	Sami Tolvanen <>,
	Masahiro Yamada <>,
	Will Deacon <>,
	Greg Kroah-Hartman <>,
	Kees Cook <>,
	clang-built-linux <>,
	Kernel Hardening <>,
	linux-arch <>,
	Linux ARM <>,
	Linux Kbuild mailing list <>,
	LKML <>,,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <>
Subject: Re: [PATCH 00/22] add support for Clang LTO

On Fri, Jul 03, 2020 at 07:42:28AM -0700, Paul E. McKenney wrote:
> On Fri, Jul 03, 2020 at 03:13:30PM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 02, 2020 at 10:59:48AM -0700, Paul E. McKenney wrote:
> > > On Thu, Jul 02, 2020 at 10:20:40AM +0200, Peter Zijlstra wrote:
> > > > On Wed, Jul 01, 2020 at 09:03:38AM -0700, Paul E. McKenney wrote:

[ . . . ]

> > Also, if C goes and specifies load dependencies, in any form, is then
> > not the corrolary that they need to specify control dependencies? How
> > else can they exclude the transformation.
> By requiring that any temporaries generated from variables that are
> marked _Dependent_ptr also be marked _Dependent_ptr.  This is of course
> one divergence of _Dependent_ptr from the volatile keyword.
> > And of course, once we're there, can we get explicit support for control
> > dependencies too? :-) :-)
> Keep talking like this and I am going to make sure that you attend a
> standards committee meeting.  If need be, by arranging for you to be
> physically dragged there.  ;-)
> More seriously, for control dependencies, the variable that would need
> to be marked would be the program counter, which might require some
> additional syntax.

And perhaps more constructively, we do need to prioritize address and data
dependencies over control dependencies.  For one thing, there are a lot
more address/data dependencies in existing code than there are control
dependencies, and (sadly, perhaps more importantly) there are a lot more
people who are convinced that address/data dependencies are important.

For another (admittedly more theoretical) thing, the OOTA scenarios
stemming from control dependencies are a lot less annoying than those
from address/data dependencies.

And address/data dependencies are as far as I know vulnerable to things
like conditional-move instructions that can cause problems for control

Nevertheless, yes, control dependencies also need attention.

							Thanx, Paul

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